Electro-optical display panels wherein information such as images or alphanumeric characters is displayed on a surface includes both active and passive devices. Active devices such as light emitting diodes, gas discharge elements, and thin film electro-luminescent elements generate their own light. Passive devices such as liquid crystal displays generate no light of their own but rather modulate incident light from natural or artificial external sources.
One particular type of liquid crystal display with which this invention is concerned is a liquid crystal in which the basic components include a liquid crystal layer sandwiched between a transparent front electrode and a reflective back electrode. In the off-state, that is with no voltage applied across the electrodes, the liquid crystal is transparent. Conversely, in the on-state, that is with a voltage applied across the electrodes, current flowing through the liquid crystal creates a turbulence that causes scattering of the light passing through the layer so that the liquid crystal appears like frosted glass. By proper illumination techniques, the viewer can distinguish between the scattering and nonscattering regions. Because the scattering increases with applied voltage, shades of grey may be obtained by varying the voltage applied to various elements. This mode of modulation of the transmissivity of liquid crystal material in response to an applied voltage is called the "dynamic scattering mode."
In practical applications, a spatially modulated electric field pattern containing image information to be displayed is impressed upon the liquid crystal by one of several methods. In one method, a matrix of individually addressable minute electrodes, each electrode forming one picture element, is formed on the backplate of the liquid crystal light valve. Such an approach is described in U.S. Pat. No. 3,824,003 issued to N. J. Koda et al. on July 16, 1974; U.S. Pat. No. 3,862,360 issued to H. D. Dill et al. on Jan. 21, 1975, and U.S. Pat. No. 4,006,968 issued to M. N. Ernstoff et al. on Feb. 8, 1977.
In another method, the image containing electric field pattern is generated from an input image directed onto a photo-conductive layer located on the backside of the liquid crystal light device. Such devices are disclosed in U.S. Pat. No. 3,824,002 issued to T. D. Beard on July 16, 1974 and U.S. Pat. No. 4,976,361 issued to L. M. Fraas et al. on Aug. 24, 1976.
Any one of these aforementioned patents may be used with the present invention and for the sake of completeness are incorporated by reference herein.
An inherent problem encountered in both the active and passive displays of the types described is that the contrast of the display is objectionably reduced by reflections from the specular display surfaces. Heretofore this problem has been solved by a light trap configuration in which the display is tilted to the viewer's line of sight so that all specular reflections from the display surface must originate from a preselected external area. That area is then blocked with a dark shield called a light trap. An example of such a configuration is disclosed in the image display device of U.S. Pat. No. 4,090,219 issued to M. N. Ernstoff et al. on May 16, 1978. In an arrangement described in FIG. 1 of that patent, a light trap comprising a planar member having a light absorbing surface extends outward from an electro-optical display panel, in this case a liquid crystal display device. The display surface is tilted to the viewers line of sight so that when the liquid crystal is in the transparent off-state, the viewer sees the dark light trap surface reflected from the backplate of the liquid crystal device with the result that the light valve appears black. When the cell is in the scattering on-state, the illumination is scattered from it toward the observer so that the activated portion of the surface appears white.
Although this display arrangement maintains a high contrast image even under illumination from highly intense sources, the arrangement has several disadvantages, one of them being that the tilt of the display surface causes objectionable foreshortening of the image displayed on the liquid crystal layer. Another disadvantage is the difficulty in designing a light trap geometry which permits locating an artificial light source needed to illuminate the display without blocking the field of view.